Inbreeding Makes Female Beetles Frisky

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What drives a female to sleep around? For red flour beetles, too
much inbreeding may do the trick, according to new research
showing that when populations of the beetles become inbred,
females get frisky.

Mating with multiple males has its downsides. When females mate
at high frequencies, they end up being less fit, and can even
sustain injuries or suffer from exhaustion. Mating with just one
male should suffice to impregnate a female, so the researchers
wondered why females
would sleep around.

Both lab-based and natural populations of these beetles often
undergo population drops that result in severely inbred
communities, study researcher Matthew Gage, of the University of
East Anglia in the U.K., said in an email to LiveScience. His
team showed that these inbred communities promote promiscuity in
female red flour beetles, and that this
promiscuous mating results in more offspring.

Breeding promiscuous females

By growing two populations of red flour beetles in the lab, a
genetically healthy population and an inbred one lacking genetic
diversity, the team was able to make the females of the group
more promiscuous. The females were quicker to mate, mated with
more males and for longer periods of time than females from a
large population that hadn't inbred and consequently put a
squeeze or bottleneck on genetic diversity.

The researchers found that when females from the inbred
population were promiscuous, they approximately
doubled the number of surviving offspring. When the
researchers turned the beetles from the healthy population
promiscuous by having each female mate with five males, there was
no similar boost in offspring survival rate.

These differences weren't caused by any fault of the males; they
were just as fertile in the inbred population as they were in the
normal population.

In addition, the females' promiscuity passed down to their
daughters, and since they had many more offspring than
nonpromiscuous females, the trait spread throughout the
population quickly.

Finding the right mate

This increased promiscuity means in a
bottlenecked population, possibly harmful genes can be found
more frequently, so being able to mate with more males could be
beneficial to females. That's because the females end up with
lots of options for which sperm fertilize their eggs; in that
way, they can avoid fertilizing their eggs with sperm from
genetically incompatible mates, such as those closely related to
them.

"Males with these kind of genetic problems will not necessarily
make good reproductive partners, as offspring might also inherit
these problems," Gage said. "If you only mate with one male, then
his sperm will be the fertilizers, so mating polyandrously allows
more intense selection of the better males and their sperm."

How this sperm selection happens isn't known, but Gage has some
ideas: "It’s possible that female beetles can smell how closely
related they are to the male, and either avoid mating with those
males, or don’t encourage sperm storage after mating with them."

The study was published today (Sept. 22) in the journal Science.

You can follow LiveScience staff writer Jennifer Welsh on
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